In short, I think you did a great job following up and your points are taken. However, the only question I have is how does the stability of this homebrew compare to others? If it is worse I really think it should be knocked down another notch or two. I can see your point about it's not possible to compare this to an EA game but the quality or stability can be compared to other homebrews. Thanks.
I highly doubt its the cartridge. Cartridges usually either work, or don't work. There's not really anything inbetween unless it has a save function.
It's likely to be a issue with the hardware. Have you recently checked any of your other titles that support the voice module? Might help isolate the problem.
Therefore it should get its well deserved A+.